1. Field of the Invention
The invention relates to a radio transmitter and especially to tuning of a transmitter used at a base station in a mobile telephone network.
2. Description of the Related Art
The invention will be discussed in connection with the GSM system, but it can naturally be used in other types of radio transceivers where applicable.
In Gaussian phase modulation used in the GSM system, a bit stream is modulated to a sine and a cosine component I, Q of a signal at a frequency here indicated by f.sub.1. The bit stream is transmitted such that every other bit is forwarded in branch I and every other in branch Q. The phase difference between signals I and Q is 90.degree.. If a signal entering one of the branches does not change, then the phase of the signal transmitted in that particular branch will not change. When the bit changes, the phase of the signal in that particular branch is shifted by 180.degree.. FIG. 1 shows signals I and Q.
Signals I and Q are mixed to the transmission frequency in an IQ modulator, the basic circuit arrangement of which is shown in FIG. 2. A local oscillator LO having a frequency f.sub.LO is connected to the modulator.
FIG. 3 exemplifies a result of up-mixing. Graph A illustrates an ideal mixing result. In this particular case, the only significant signal of the spectrum of the mixing result is at frequency f.sub.LO +f.sub.1. Low-frequency signals can be seen at frequency f.sub.LO of the local oscillator LO and at image frequency f.sub.LO -f.sub.1 of frequency f.sub.1. With some other bit combination, the latter frequency may be an effective frequency. Due to non-linearity of the components, the frequency response is in practice much more complicated, as illustrated by graph B in FIG. 3. Compared with ideal result A, the payload signal of result B is attenuated and the undesired mixing results are emphasized. Graphs C and D of FIG. 3 will be discussed later.
FIG. 4 is a block diagram of that part of a transceiver unit at a base station of the GSM system which is essential to the invention. A baseband unit 1 comprises a digital part 10, which divides digital data D into bit streams D.sub.I and D.sub.Q, which correspond to signals I and Q. The bit streams are converted into analogue signals I and Q with D/A converters 12 and 14. The output signals of the converters are filtered with low-pass filters (not shown) and they are supplied through adjustable amplifiers 16 and 18 to an IQ modulator 32. From the IQ modulator 32, the signal passes through an amplifier 40 to an antenna 50. At a multichannel base station, several transceiver units are connected to a common TX antenna with a transmission adder (not shown).
An RX signal of a TRX unit at a base station of the GSM system is received with an antenna 52. At a multichannel base station, the antenna 52 is followed by a receiving multicoupler (not shown). The signal is supplied further through an amplifier 54, a mixer 56, a band-pass filter 60 and an amplifier 62 to a Received Signal Strength Indicator (RSSI) 22 in the baseband unit 1. It has been assumed above that the RSSI indicator 22 comprises an A/D converter. Otherwise, a separate A/D converter will have to be arranged before the RSSI indicator 22. With a suitable arrangement, antennas 50 and 52 can also be one and the same antenna. The reception mixer 56 is controlled by a local oscillator 58. The IQ modulator 32, in turn, is controlled by a transmission oscillator 34.
In this example, the IQ modulator mixes directly to the transmission frequency, but in the invention the IQ modulator may just as well mix first to an intermediate frequency, whereby there is a separate intermediate frequency stage between the IQ modulator and the transmission antenna 50.
For internal testing and adjustment of the TRX unit, the signal supplied to the transmission antenna is sampled with a directional coupler 42, and the sample is supplied through a TRX loop mixer 44 to a reception branch 54-62 with a directional coupler 48 for RSSI calculation. In the prior art transceiver equipment, the problem described in connection with FIG. 3 is solved by adjusting the equipment during manufacture such that a proportion of undesired signal components is minimal. An amplitude error can be corrected by adjusting the gain of amplifiers 16 and 18, and a phase error e.g. by adjusting the symmetry of the IQ modulator 32 in accordance with FIG. 2. This tuning action takes time. In addition, a tuning action performed does not protect against possible later drifting of component values.